Simple breakdown of Civil War formations for both the North and South. These numbers are approximations and would be from the day of enlistment. During the Civil War most units were not fully staffed. It is saft to assume that when reading about battle accounts you can take the numbers below and reduce them 30% to 50%.
100 men and commanded by a Captain.
1000 men made up by 10 companies. Commanded by a Colonel.
3,000 to 6,000 men made up by 3 to 6 Regiments. Commanded by a Brigadier General.
The South would often times use more regiments to make up their brigades then the North giving them larger Brigades. There may also be at times an Artillery or Cavalry attached to a Brigade.
9,000 to 36,000 men made up from 2 to 6 brigades. Commanded by Major General.
The reason for the large span in numbers of a division comes from different make ups between the North and South. The North tended to have smaller divisions composed from 3 or 4 Brigades. The South would often use more Brigades (4 to 6).
27,000 + men made up from 2 to 4 divisions. In the North they are commanded by a Major General, in the South a Lieutenant General.
Again, differences in the North and South setups creates large number gaps here as well.